The present invention relates to power transmission devices for rotary power sources such as internal combustion engines and electric motors in applications variably driving inertial loads such as vehicles.
Typical power sources such as electric motors and internal combustion engines have performance characteristics that are greatly dependent on rotational speed (RPM). Principal characteristics include, available torque, power, and efficiency, the efficiency being a function of not only the speed but also the operating torque or power. Variable speed electric motors are typically characterized by a linear torque-speed relation, the speed, which can be bidirectional, being limited by applied voltage, the torque, which also can be bidirectional, being limited by applied current. Thus electric motors are typically quite flexible in terms of operating speed; however, the efficiency is greatly dependent on speed, being greatest within a fairly narrow range of speeds. Internal combustion engines have more severe performance limitations in that they are not only normally unidirectional, but they are substantially inoperative below certain speeds, and the available torque falls off below an intermediate speed (the torque peak). The efficiency of internal combustion engines is also greatly dependent on speed. Moreover, the design of the engines as well as the motors involves serious compromises between maximum performance and efficiency at optimum speeds and flexibility over a wide range of speeds.
Loads such as vehicles present distinctly different load components of inertia, rolling resistance, wind resistance, and climbing resistance which, in combination with the operating characteristics of the power source, dictates a need for a variable ratio transmission connected between the source and the load. Vehicle transmissions are well known, mainly for use with internal combustion engines, in several basic configurations including hydraulic torque converters, torque converters or hydraulic couplings in combination with automatically shifted gearing, and manually shiftable gear trains together with manually operated clutches. As discussed below, each of the transmissions of the prior art is subject to significant disadvantages.
Manually operated transmission gearing and clutches of conventional construction offer high efficiency and more direct control of the load than automatic transmissions. However, they are subject to a number of disadvantages, such as:
1. They require higher driving skill levels and attention, both for reasonably smooth operation and for maintaining reasonably efficient gear ratios; PA1 2. They normally require significant interruptions in power from the engine during gear changes; and PA1 3. They have a very limited number of gear ratios (typically four or five) which must therefore be widely spaced in a compromise between low speed acceleration and climbing ability as against high speed efficiency, with intermediate speeds often being (especially during climbing) such that one ratio is excessively low, yet the next ratio is too high for satisfactory performance and/or efficient operation of the power source. PA1 1. Significant skill is required for selecting a desired gear using the disclosed linear arrangement of detent positions, for simultaneously interrupting power during up-shifts, and for selecting appropriately efficient gear ratios; PA1 2. The gearing is subject to loading with the slidable gear only partially meshed with the higher of two beveled gears, resulting in potential gear failure; and PA1 3. The power interruptions needed for reliably avoiding gear damage (typically 0.3 second to 0.6 second) are undesirable as adversely affecting both performance and comfort.
Automatic transmissions having torque converters combined with the usual planetary gearing are easy to operate and are free from significant power interruptions during gear changes. However, they are excessively heavy, bulky, expensive to provide, and they are significantly inefficient, often requiring dedicated cooling apparatus.
Variable ratio frictional power transmission is also known, by means of wheels that frictionally engage rotating cone-or disk-shaped members at varying radii, or by V-belt drives wherein at least one sheave has variably spaced flanges. These devices are subject to excessive wear and limited torque capacity, and the belt drives are significantly inefficient.
U.S. Pat. No. 1,111,551 to Adams discloses transmission gearing in which a cone-shaped collection of beveled gears, each of the beveled gears having one-way clutch engagement with a common output shaft, another gear being slidably keyed to an independent shaft and shiftable along the independent shaft in engagement with selected ones of the beveled gears. The gearing of Adams, while providing a greater selection of ratios within a given physical space than do conventional manual transmissions, is nevertheless subject to a number of disadvantages. For example:
Thus there is a need for a variable ratio transmission that provides a wide range of closely spaced gear ratios, that is operable without significant power interruptions, and that is easy to use.